PCB Blog - Solutions to Power Supply Noise in High Frequency PCB Design

In high-frequency PCB board, the most important type of interference is power supply noise. By systematically analyzing the characteristics and causes of power supply noise on high-frequency PCB boards, and combining them with engineering applications, some very effective and simple solutions are proposed.

Power supply noise refers to the noise generated by the power supply itself or induced by disturbances. Its interference is manifested in the following aspects:

1) Distributed noise is caused by the inherent impedance of the power supply itself. In high-frequency circuits, power supply noise has a greater impact on high-frequency signals. Therefore, a low-noise power supply is required first. Clean ground and clean power are equally important.

Ideally, a power supply has no impedance, so there is no noise. However, the actual power supply has a certain impedance, and the impedance is distributed over the entire power supply, so the noise will also be superimposed on the power supply. Therefore, the impedance of the power supply should be reduced as much as possible, and there are special power supply layers and ground layers. In high-frequency circuit design, it is generally better to design the power supply in the form of a layer than in the form of a bus, so that the loop can always follow the path of the impedance. In addition, the power board must provide a signal loop for all signals generated and received on the PCB, which can reduce the noise of the signal loop.

2) Common mode field interference. It refers to the noise between the power supply and the ground. It is the interference caused by a power supply caused by the loop formed by the interfered circuit and the common-mode voltage caused by the common reference plane. Its value depends on the relative relationship between the electric field and the magnetic field. Strong or weak.

On this channel, the drop in Ic causes a common-mode voltage in the series-connected current loop, affecting the receiver section. If the magnetic field dominates, the value of the common-mode voltage developed in the series ground loop is:

3) Differential mode field interference. Refers to the interference between the power supply and the input and output power lines. In the actual PCB design, the author found that its proportion in the power supply noise is very small, so it can not be discussed here.

4) Power line coupling. It refers to the phenomenon that after the AC or DC power line is subjected to electromagnetic interference, the power line transmits the interference to other devices. This is the interference of power supply noise to high-frequency circuits indirectly. It should be noted that the noise of the power supply is not necessarily generated by itself, but may also be the noise induced by external interference, and then superimpose this noise with the noise generated by itself (radiation or conduction) to interfere with other circuits or devices.

Countermeasures to Eliminate Power Supply Noise Interference

According to the different manifestations and causes of power supply noise interference analyzed above, the conditions under which it occurs can be targeted, and the interference of power supply noise can be effectively suppressed. The solutions are:

1) Pay attention to the through holes on the board. The vias require etched openings in the power supply layer to make room for the vias to pass through. If the opening of the power layer is too large, it will inevitably affect the signal loop, the signal will be forced to detour, the loop area will increase, and the noise will increase. At the same time, if some signal lines are concentrated near the opening and share this section of the loop, the common impedance will cause crosstalk.

2) The connecting wire needs enough ground wires. Each signal needs to have its own dedicated signal loop, and the loop area of the signal and the loop should be as small as possible, that is, the signal and the loop should be parallel.

3) Place a power supply noise filter. It can effectively suppress the noise inside the power supply and improve the anti-interference and safety of the system. And it is a two-way radio frequency filter, which can not only filter out the noise interference introduced from the power line (to prevent the interference of other devices), but also filter out the noise generated by itself (to avoid interfering with other devices), and interfere with the series mode and common mode. both inhibited.

4) Power isolation transformer. Separating power loops or common-mode ground loops of signal cables can effectively isolate common-mode loop currents generated at high frequencies.

5) Power regulator. Regaining a cleaner power supply can greatly reduce the noise level of the power supply.

6) Wiring. The input and output lines of the power supply should not be placed on the edge of the dielectric board, otherwise, radiation will easily occur and interfere with other circuits or devices.

7) The analog and digital power supplies should be separated. High-frequency devices are generally very sensitive to digital noise, so the two should be separated and connected together at the inlet of the power supply. If the signal is to span the analog and digital parts, a loop can be placed where the signal crosses to reduce the loop area. Figure 4.

8) Avoid separate power supplies overlapping between different layers. Try to stagger them, otherwise, the power supply noise can easily couple through the parasitic capacitance.

9) Isolate sensitive components. Some components such as phase-locked loops (PLLs) are very sensitive to power supply noise and should be kept as far away from the power supply as possible.

10) Place the power cord. To reduce signal loops, noise reduction can be achieved by placing power lines next to signal lines.

11) In order to prevent the interference of power supply noise to the circuit board and the accumulated noise caused by external interference to the power supply, a bypass capacitor can be connected to the ground in parallel on the interference path (except radiation), so that the noise can be bypassed to the ground to avoid interfering with other equipment and devices.

Power supply noise is directly or indirectly generated from the power supply and interferes with the circuit. When suppressing its influence on the circuit, a general principle should be followed, that is: on the one hand, it is necessary to prevent the power supply noise as much as possible. The influence of the circuit, on the other hand, should also minimize the influence of the outside world or the circuit on the power supply, so as not to deteriorate the noise of the power supply on the PCB board.